It reboots as in turns completely off and then on? Or does it /respring/ which just reloads the UI? Sounds like you need to restore and start all over. Set the phone up as a /new/ profile in iTunes.. not from a back-up in case there is corrupt code or an incompatible app causing crashes.

If it's still having problems after restoring as a new device, you must have some type of hardware problem. When you say cutting out is it still rebooting, or it it just dropping calls. Remember, if you need a phone the iPhone is not the best choice.

Yes still rebooting, phoned Tesco who I bought it off after the restore and I was on the phone for 19 mins and it didn't happen, so fingers crossed it might be ok. If not it needs to be sent off to be looked at. If I do a full DFU restore will they be able to tell it was JB?

Just an update, had to send it off for repair, couldn't be repaired acceptably and they sent me a new 3GS as replacement, no JB though as running 4.0.1 and have just updated to 4.1 decided i only wanted JB for sat nav so gonna wait and probably buy official DL of TomTom.